1. Field of the Invention
This invention relates to an exhaust gas treatment device for injection molding machines which prevents injection material from escaping through the vent, deaerates and effectively treats the exhaust gas from the vent.
2. Description of the Prior Art
Conventional exhaust gas treatment devices are known to be directly connected to the vent of the injection molding machines or installed separately from the injection molding machine. They either release the exhaust gas directly into the open air or extract the gas through the vent by applying negative pressure to the vent by a vacuum pump. The conventional exhaust gas treatment device, however, has the following drawbacks.
In a system where the gas is extracted by the vacuum pump, the gas can be extracted satisfactorily, but when the screw stops rotating and the molten resin expands, part of the molten resin cannot be prevented from escaping through the vent depending on the kind of resin used and the quantity of gas contained in the melted resin even if the vacuum degree in the vent is decreased; the tarlike components contained in the gas exhausted from the vent adhere to the inner wall of the exhaust gas treatment device, the heater and the catalysts and when the device is restarted after some interval the sticking tar is directly heated and oxidized, raising the temperature within the device to an abnormally high level, which may result in a explosion of the device. Thus, it has been necessary to provide the exhaust gas treatment device with an apparatus for removing explosive gases. The causes for this explosion can be enumerated as follows.
(a) Since the gas exhausted from the vent is directly and continuously treated, the concentration of oxygen contained in the gas becomes low and the combustion of the gas becomes incomplete. PA0 (b) The tar contained in the untreated gas adheres to the catalysts so that the concentration of gas becomes high and an abnormal heat generation occurs when the device restarts its operation.
Since in the conventional device the treated hot (about 350.degree.) gas is directly released into the open air or drawn into the vacuum pump, there is a possibility of causing accidents when the hot gas is exhausted into the atmosphere or sucked into the pump.
To solve these problems the inventor of this invention formerly proposed to install a deodorizing device and a mist separator between the vent and the vacuum pump. However, this provision poses the following problems: The pressure in the vent must be sharply reduced to a level of -680 to -720 mm Hg immediately (within two seconds) after the screw has begun to rotate. However, provision of the deodorizing device between the vent and the vacuum pump requires a pump with large capacity which will result in an increase in the cost of investment and the running cost. Further, when a large amount of air is abruptly sucked, the vacuum pump blows up a large amount of water. This will shorten the life of the vacuum pump and the water blown up will generate secondary pollution.